magit-diff-none: inherit from default face

Previously we inherited from `diff-context' but because that does not
set a background color in the default them that meant that unchanged in
diffs were highlighted.  Since we by default use a rather ugly color for
that purpose, that was not acceptable.

See issue #1042.
Revert "magit-diff-none: inherit from default face"

This reverts commit 93a504b6c1ec12ac82612003dd4fcbc4d5aa8cab.